Could clustering be used to parse pdf documents to get headings and titles?
Yes, you could use clustering: Encode your features as a feature vector and feed it into a clustering algorithm (see Finding Groups in Data for a comprehensive description of these). You could use agglomerative clustering, which would give you groups of similar items; perhaps different level headings will be clustered together.

Alternatively you could try a decision tree, something like ID3, which would also be suitable; for this you'd need some annotated training data, though. But with a small amount of data you might solve it, if your items are clearly separated.